[DWARFv5] Allow ".loc 0" to refer to the root file.

DWARF v5 explicitly represents file #0 in the line table.  Prior
versions did not, so ".loc 0" is still an error in those cases.

Differential Revision: https://reviews.llvm.org/D48452

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335350 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed